Vision Research 51 (2011) 1099–1108 Contents lists available at ScienceDirect Vision Research journal homepage: www.elsevier.com/locate/visres A comparative study on the visual adaptations of four species of moray eel ⇑ Feng Yu Wang a,b,1, Meng Yun Tang a,1, Hong Young Yan a, a Sensory Biology Laboratory, Marine Research Station, Institute of Cellular and Organismic Biology, Academia Sinica, Jiaoshi, I-Lan County 26242, Taiwan b Taiwan Ocean Research Institute, Taipei 10622, Taiwan article info abstract Article history: The goal of this study was to investigate how the eyes of different species of moray eel evolved to cope Received 18 November 2010 with limitations to vision imposed on them by the photic environments in which they reside. The com- Received in revised form 22 February 2011 parative retinal histological structures and visual pigment characteristics including opsin gene sequences, Available online 6 March 2011 of four species of moray eel inhabiting diverse habitats (i.e., shallow-water species, Rhinomuraena quae- sita and Gymnothorax favagineus, and deep-sea species, Gymnothorax reticularis and Strophidon sathete) Keywords: were examined. The histological sections showed that retinal layer structures of R. quaestia are signifi- Moray eel cantly different from those of the other three species which likely reflects the effects of distribution depth Microspectrophotometry (MSP) on the structures. The maximal absorbance wavelength (kmax) of photoreceptor cells, as measured by kmax Visual characteristics microspectrophotometry (MSP), showed a close correlation between the kmax and the intensity/spectral Opsin gene quality of the light environment where each species lives. The spectra-shift, between shallow and deep- sea species, observed in the rods cells results from amino acid substitution in Rh1 gene, while that in cones most likely results from differential expression of multiple Rh2 genes. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Vision begins when photons are absorbed by photoreceptors in the retina. Two types of photoreceptors are found in most vertebrate The solar irradiance measured at depth in natural waters is retinas – rods and cones. Rods mediate scotopic vision and generally influenced by the absorptive characteristics of the water as well have long, cylindrical outer segments. Cones mediate photopic, high as the time of the day, suspended particle, nutrient load, phyto- acuity vision, and usually have shorter, more conical outer seg- plankton and zooplankton concentrations. Due to these factors, ments. They can exist as single cells or into coupled groups as dou- the photic environment of aquatic organisms exhibits a great bles or even triples (Sandström, 1999). Both types of photoreceptors diversity of irradiant and optical conditions. In order to adapt to contain visual pigments, which are composed of an opsin protein the wide extent of specific photic environments, such as those and a chromophoric group, either 11-cis-retinal (based on vitamin found in estuaries, coastal, shallow, deep-sea, rivers and lakes, A1) or 11-cis-3-dehydroretinal (based on vitamin A2). In vertebrates, fishes have evolved various visual system characteristics allowing there are five opsin gene families giving rise to the visual pigments them to operate under different types of photic conditions (Loew (Yokoyama, 1994, 1995, 1997; see Bowmaker & Loew, 2008). Rh1 is & McFarland, 1990). As solar radiation penetrates clear blue oce- expressed in the rods and yields vitamin A1-based visual pigments anic water, the shorter wavelengths (i.e., blue light; ca. 400– having kmax from 460 to 530 nm (Yokoyama, 1997). The vitamin 500 nm) are absorbed less than longer wavelengths resulting in a A1-based visual pigments found in cones formed by the other four narrowing of the visible spectrum at depth with the peak of the expressed opsin genes are a long- to middle-wave class (LWS) max- downwelling light being in the region of 435 nm (Kirk, 1983). In imally sensitive in the red–green spectral region from about 490– coastal and fresh water the increase in dissolved organics, i.e., 570 nm, a middle-wave class (RH2) sensitive in the green from the so-called ‘‘Gelbstoff’’ and scattering particulates shifts the about 480–535 nm, a short-wave class (SWS2) sensitive in the transmission maximum to longer wavelengths (Jerlov, 1968). blue–violet from about 410–490 nm and a second short-wave class Therefore, in clear water, the photic environment exists as a (SWS1) sensitive in the violet–ultraviolet from about 355–440 nm blue–green color, while the spectrum of the ambient light in coast- (Bowmaker, 2008; Bowmaker & Loew, 2008; Bowmaker, Semo, al and lake waters would be more in the green to orange wave- Hunt, & Jeffery, 2008; Ebrey & Koutalos, 2001; Yokoyama, 2000; length range (McFarland, 1986; Morel, 1980). Yokoyama & Yokoyama, 1996). A number of visual system adaptations allow fish to cope with ⇑ Corresponding author. Fax: +886 3 9871035. the constraints imposed by a habitat’s specific photic environment. E-mail address: [email protected] (H.Y. Yan). First, variations in eye and retinal structure allow some fishes to 1 These authors contributed equally. exploit different habitats and niches more effectively (Bowmaker, 0042-6989/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.visres.2011.02.025 1100 F.Y. Wang et al. / Vision Research 51 (2011) 1099–1108 1990, 1995; Collin, 1997). For example, fishes that live in deep-sea 2002); (2) the deep-water group, consisting of the dusky-banded environments have adaptations that address the problems of low moray, Gymnothorax reticularis (depth range: 30–200 m) and the light intensity such as larger eyes or a tapetum which reflects light slender giant moray, Strophidon sathete (depth range: 1–300 m), back (Nicol & Somiya, 1989; Warrant & Locket, 2004). There may which live in sand–muddy sediment (Randall, Allen, & Steene, also be longer outer segments that increase the probability of pho- 1990; Smith & Bohlke, 1997). Since the habitats of these two ton capture or banked retinas (see McFarland, 1991). The problems groups of moray eels differ so much in their respective photic envi- of the spectral shifts in background space light due to depth and ronments, comparisons of the differences between these two changes in water quality have been addressed by altering the groups could provide useful information to delineate how moray absorptive properties of the visual pigments either by amino acid eels evolved to cope with the environmental constraints in terms alterations of visual pigment opsins that create visual pigments of light conditions. more appropriately ‘tuned’ to the visual tasks present, or by alter- In this study, histological methods were used to measure the ing the expression pattern of the opsin genes, or both (Bowmaker thickness of each retinal layer with the expectation that increases et al., 2008; Carleton & Kocher, 2001; Cottrill et al., 2009; Parry in photoreceptor and outer nuclear layer thicknesses would be et al., 2005; Shand, Hart, Thomas, & Partridge, 2002; Shand et al., associated with the dim light condition. Second, the absorption 2008). There is also the possibility of switching chromophore class spectra of the photoreceptor cells were obtained by microspectro- (vitamin A1- to vitamin A2-based) or employing some kind of pho- photometry (MSP). Finally, the opsin genes from these four moray tosensitizer as has been found for some deep-sea species (see Bow- eel species were cloned and sequenced. The combination of these maker & Loew, 2008). data allow us to speculate on how moray eels have adapted to their Numerous studies have documented the changes associated photic environments. with the retinas and visual pigments of fishes inhabiting different photic environments. For visual pigments, the findings have been 2. Materials and methods interpreted in the context of two hypotheses. The Sensitivity Hypothesis states that for maximizing the brightness contrast of 2.1. Samples collection a target against its background a single photoreceptor visual pig- ment k should be located close to the maximum of the down- max The moray eel species used in this study were obtained in a welling space light to maximize quantum catch. Thus, the k of max variety of ways. Specimens of R. quaesita (ribbon eel) were im- rod visual pigments shifts to shorter wavelengths as habitat depth ported from Southeast Asian waters via a vendor in Singapore. G. increases (see Bowmaker, 2008). The Contrast Hypothesis states favagineus (laced moray) were bought in Bi-Sha Fishing Harbor, that two visual pigments are necessary for maximizing chromatic Keelung, Taiwan, where they were caught with plastic tubing traps (i.e., color) contrast – one with its absorbance matched to the back- at a depth of approximately 30 m around Peng-Hu Archipelagos, in ground space light and the other offset from the background so as the middle of Taiwan Strait. G. reticularis (dusky-banded moray) to maximize the difference in the background and target chroma- and S. sathete (slender giant moray) were caught by bottom trawl- ticities (see Bowmaker, 2008). ers from depths of 50–800 m and landed in Da-Si Fishery Harbor, I- Numerous fish groups from different habitats have been exam- Lan, Taiwan. All specimens were kept in a tank with running sea- ined for their visual pigment complement and their retinal struc- water (temperature of 25–28 °C) under natural light cycle at the ture. However, few have been conducted on members of eel Marine Research Station, Institute of Cellular and Organismic Biol- families, including freshwater eels and the conger eels. To adapt ogy, Academia Sinica, Taiwan. They were fed with fish meat ad libi- to the deep-sea environments, freshwater eels (Anguilla spp.) and tum three times a week until use. The animal use protocols used in conger eels (Conger spp.) possess photoreceptors with a blue- this study were approved by Academia Sinica Institutional Animal shifted kmax (Archer & Hirano, 1996; Denton & Walker, 1958; Shap- Care and Use Committee (No.